Spread pecan pieces on a sheet pan in a single layer and toast in a 325-degree
oven just until they are lightly browned and aromatic. Spread on parchment or
wax paper to cool.

In a medium bowl, whisk together flour, baking powder, salt and brown sugar,
just to combine.

In a separate bowl, whisk together milk, eggs and vanilla. Pour half of the
liquid mixture into the dry ingredients and whisk; repeat with remaining half.
Do not overmix. Stir in melted butter, then fold in diced plums and 1/2 cup
toasted pecans. Spoon batter onto preheated waffle iron and cook until golden.